Such systems measure, with precision, the angular deviation of the direction of an intercepted target from the boresight axis of an optical tracker. When the target is mobile, such as aircraft or a winged or ballistic missile, the system follows the movement of the vehicle while maintaining optical contact in the infra-red, visible or ultra-violet light range. The optical field covered by a tracking objective is generally limited in the corresponding focal plane to a useful detection area of rectangular shape centered on the optical or boresight axis. Thus, there corresponds to each remote target in the field of observation, radiating light rays toward the system within a predetermined spectral band, a spot image in the detection area whose positioning with respect to the center of that area defines the off-boresight error. The cartesian co-ordinates of this point in the focal plane represent in this manner the spatial co-ordinates of the target corresponding to elevation and azimuth.
It is understood that the sighted target comprises a light source of continuous emission having a quasi-omnidirectional radiation pattern so as to enable easy acquisition by optical sight and continuous reception of a fraction of the power radiated toward the optical system whatever the angular off-boresight error in the tracking field.
A system of the type with which the present invention is concerned proceeds by scanning the useful area of the focal plane forming the image of the spatial field sighted by the optical device. Localization of the target in cartesian co-ordinates is obtained by dividing this rectangular useful area into a plurality of juxtaposed elementary strips each having the same length as a dimension of the image rectangle, for example the dimension considered as the abscissa axis (X). The combined width of these juxtaposed strips equals the other dimension of the useful area, i.e. the dimension along the axis of the ordinates (Y). Exploration of the useful area is effected by mechanical scanning means affording a high degree of resolution. According to known techniques, e.g. as disclosed in British Pat. No. 1,176,003, this mechanical scanning proceeds by sequentially moving a plurality of parallel slots in front of and close to the focal plane in a direction perpendicular to the principal dimension of the elementary strips.
A light-source-tracking device of this type enables, by using a linear array of rectangular detectors, a determination of a first spatial co-ordinate, such as the azimuth, by the rank of the excited detector and a second spatial co-ordinate, the elevation in this case, by analysis of the time of occurrence of the detected signal which is time-modulated by the scanning slots. Determination of these spatial co-ordinates is effected by electronic processing.
The performance of such a system is particularly satisfactory from the point of view of filtering out parasitic light sources (sun, clouds, etc...) which are usually of larger dimensions than the detected target. There results therefrom a great improvement of the useful signal-to-noise ratio, the noise being formed essentially by continuous (d-c) components and variable signals due to parasitic sources. However, this technique of operation has its optimum performance limited by the fact that an increase in frequency selectivity of the treatment circuits diminishes not only the noise but also the accuracy of measuring the abscissa (X) of the target whereas a decrease in selectivity has the opposite effect.